Verdure: T is for turfgrass, and P is for Poa

Auburn University researchers evaluated the effects of phosphorus on Poa annua growth and seedhead production.

Hate it, love it or tolerate it, Poa annua is a persistent plant in the turfgrass landscape. In fact, it is the only nonnative flowering plant species found on every continent in the world. Now that is a plant that can adapt.

Given its ubiquity in the landscape, turfgrass researchers have been studying Poa annua for a long time. Early work that examined soil characteristics found that Poa annua was negatively affected by high soil phosphorus or by low soil pH. That work was often conducted with a goal of keeping Poa annua in the landscape. However, in the southeastern United States, Poa annua is usually considered a weed, and research is conducted with a goal of reducing Poa annua populations.

In 2010, scientists at Auburn University revisited phosphorus fertilization effects on Poa annua, this time with a goal of determining whether phosphorus could be used to control Poa annua rather than to encourage its growth.

The researchers grew pots of Poa annua from seed, with the seed placed in three different soil types: a USGA-type sand/peat greens mix (90/10 v/v), a clay soil, and a loamy sand soil. Before seeding, five rates of phosphorus (applied as triple superphosphate) were mixed into the pots of soil: 0, 0.5, 1.0, 2.0 and 4.0 pounds of phosphorus/1,000 square feet (0, 25, 50, 100 and 200 kilograms/hectare).

After germination and two weeks of growth, plants were thinned so that there were five Poa plants in each pot, and they were allowed to grow until seedhead production and maturity. As the Poa annua grew, the number of days until the emergence of a seedhead was recorded. Once a seedhead matured, it was clipped, and the following information was collected for each seedhead: 1) number of branches on a seedhead, 2) number of seed on each branch, and 3) weight of the seed from each branch.

After all the seedheads were harvested from a pot, the total number of seedheads, branches and seed plus the weight of those seed were calculated for each soil type/phosphorus rate treatment. Randomly selected seed were then used in an experiment to measure percent germination.

All of that clipping, sorting and weighing revealed that soil type and phosphorus fertilization each affected the growth of Poa annua. There were generally no differences attributed to growing Poa in the two native soils, but Poa annua growing in the greens mix was shorter and had fewer seedheads, seedhead branches and seed, and it took longer for the plants to produce a first seedhead. These differences were likely due to the overall lower initial soil fertility of the greens mix. Although the plants were fertilized uniformly during the experiment, background soil fertility was not adjusted at the start of the experiment.

Shoot weight and total seed weight increased as the phosphorus rate increased. Additionally, as the phosphorus rate increased, seedheads appeared earlier. When no phosphorus was applied, it took 127 days for a seedhead to appear, compared with 119 days to seedhead emergence when any phosphorus was applied.

All other measured variables — number of seed, weight of that seed, number of seedhead branches and germination of harvested seed — were largely unaffected by soil type or phosphorus rate. In general, Poa annua plants that were grown in soil with a high sand content and received no or lower rates of phosphorus were smaller and took a week longer to produce a visible seedhead. These results could affect decisions about growth regulator applications intended for Poa annua seedhead suppression.

Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.